The proper establishment of synapses during development and their maintenance throughout adulthood is critical for complex brain functions. Synaptic dysregulation is associated with many neurological diseases such as schizophrenia, Fragile X and Alzheimer's. An understanding of how synapses are altered in disease states will first require an understanding of the molecules and pathways responsible for the establishment of synapses in the central nervous system during brain development. In vertebrates, the majority of excitatory synapses are formed on dendritic spines, tiny, actin-rich structures that protrude from the dendritic surface and act as the platform for presynaptic input. The dynamic remodeling of the actin cytoskeleton drives dendritic spine development and synapse formation. However, the cellular mechanisms responsible for actin organization and remodeling during postsynaptic development are not fully understood. Tropomodulins (Tmods) are a multi-domain family of proteins that cap the minus end of actin filaments; thereby blocking monomer exchange at the minus ends, inhibiting depolymerization, and stabilizing actin-based structures. Tmods are highly expressed in the central nervous system and genetic knock-out of Tmod2 in mice leads to behavioral and synaptic defects. However, the mode of action and underlying cellular mechanisms responsible for such phenotypes are unknown. Likewise, the isoform specific functions as well as the actin-regulatory domains of Tmod responsible for proper neuronal function have not been investigated. Thus, if and how Tmods play a role in actin remodeling during dendritic spine development and synapse formation remain unknown. Our preliminary data reveal that Tmods are expressed in both developing and mature dendritic spines, where in mature spines, Tmods concentrate in a region that contains relatively stable actin filaments. We find that Tmod protein levels increase in the hippocampus during postnatal development, a time when extensive synaptogenesis and refinement occurs. We hypothesize that minus end capping of actin filaments by Tmod is essential for the development and stability of dendritic spines during postsynaptic development, thereby allowing the structural modification of dendritic spines that occurs during synapse formation. In Specific Aim 1, we will knockdown Tmods in developing primary hippocampal neurons and use immunocytochemistry, live cell imaging, and electrophysiology to determine the function of Tmods during spinogenesis and synapse formation. In Specific Aim 2, we will mutate the Tmod minus end capping domain in order to investigate the role of Tmod minus end capping during postsynaptic development. By understanding the mechanisms by which Tmod minus end capping regulates the actin cytoskeleton within dendritic spines, this work will aid in our long term goal of understanding the mechanisms by which excitatory synapses are established, maintained and dysregulated in disease states.